Industrial robots, or modular robots, are conventionally designed according to a specific design scheme for how joint actuators, such as electric motor units, and structural elements, such as joint modules, are connected to form a robot arm, or a manipulator. There are various kinds of known modular robot designs and various designs of joints and arm modules in prior art.
EP-A1-1 930 129 describes an articulated manipulator comprising a plurality of arm bodies in series, and joint parts rotatably connecting two adjacent arm bodies of the plurality of arm bodies, where the joint parts are provided such that rotation axes of the adjacent joint parts intersect each other at right angles.
U.S. Pat. No. 4,527,945 discloses a swivelling handle for a robot arm including three reduction gear assemblies, each assuring the control of movement about three axes of rotation, where one of the axes intersects both of the other two.
WO-A1-2005/061189 describes a wrist unit for connection to a robot arm of an industrial robot. U.S. Pat. No. 4,766,775 discloses a modular robotic manipulator comprising articulating modules. U.S. Pat. No. 4,900,997 describes a device to guide an object around two axes of rotation. EP-A1-0 612 591 discloses a motorized rotary joint for robots.
WO-A2-2007/099511 discloses a programmable robot system comprising a robot provided with a number of individual arm sections, where adjacent sections are interconnected by a joint.
EP-A2-0 233 712 discloses an industrial robot having integrated joint drive modules assembled in series to form an arm. The joint modules are of two basic types, roll joints and pitch joints, and the typical robot arm configuration includes a series of alternating pitch joints and roll joints terminating in a tool at the distal end of the arm. Each joint, or joint module, is independently powered by its own motor which is attached to the joint. Each pitch joint is disposed with its axis of rotation substantially perpendicular to the roll joint axes, and each pitch joint is offset from the longitudinal axis of the robot arm extended with the roll joint axes collinear. Said offset permits the robot arm to be folded back on itself or to be fully extended.
However, the above-mentioned industrial robot designs have various kinds of drawbacks. Some designs result in a bulky arm structure, in one or more directions, which is not as compact as desired. Some designs have limitations in the dynamic performance, e.g. because of high inertia caused by the weight the arm modules or joints of the arm, and have limited joint strokes. This impairs or limits the movement of the robot arm.